Hillary Clinton is fighting a rearguard action to avoid further damage to her standing with the crucial Jewish vote in her campaign for a Senate seat after a claim that she once used an ethnic insult against one of Bill Clinton's aides.

A book out today claims that 26 years ago she called Paul Fray, her husband's campaign manager at the time, a "fucking Jew bastard".

Mrs Clinton denied the allegation and pressed the president into supporting her, but the author, Jerry Oppenheimer, said: "Three witnesses have now publicly acknowledged that she said it."

The Jewish vote accounts for 14% of the New York state electorate and is being courted assiduously by Mrs Clinton and her Republican opponent, Rick Lazio.

Mr Clinton won 80% of the bloc - traditionally sympathetic to the Democrats - in the last presidential election, but recent polls show that his wife has the support of only about 50%.

"I have never said anything like that, ever," Mrs Clinton said. "I have in the past certainly, you know maybe, called somebody a name. But I have never used an ethnic, racial, anti-Semitic, bigoted, discriminatory, prejudiced, accusation against anybody. I've never done it. I've never thought it."

Mr Clinton, who is trying to bring about a Middle East peace deal at Camp David, backed her up. "I was there on election night in 1974 and the charge is simply not true," he said.

"She might have called him a bastard, I wouldn't rule that out. She's never claimed that she was pure on profanity. But I've never heard her tell a joke with an ethnic connotation. She's so fanatic about it. It's not in her."

Referring to a "vast rightwing conspiracy" in previous elections, he said: "This is part of a pattern. They couldn't defeat me politically and they can't defeat her politically so they go after us personally."

Accounts of the dispute on election night 1974, when Mr Clinton failed to win a congressional seat representing Arkansas, have appeared before without the ethnic insult.

Those present at the inquest into the defeat were Mr Clinton, his then-girlfriend Hillary, his campaign manager Paul Fray, and Mr Fray's wife, Mary Lee. Neill McDonald, a campaign worker, was just outside the room and claims to have heard everything.

The Frays and Mr McDonald are the three witnesses on whom Mr Oppenheimer, a former reporter on the National Inquirer tabloid, relies for the passage in his book State of a Union: Inside the Complex Marriage of Bill and Hillary Clinton.

Mr Fray, 57, said yesterday: "I was a little defensive about it. I looked to the floor thinking 'How do I respond?' I didn't mind being called a son-of-a-bitch, but when it came to attacking my culture, that's a whole 'nother ballgame.

"You've got to understand it was the heat of the moment. We knew we had lost. It was a case of people lashing out at one another and it just got to that point."

Mr Fray is actually a Southern Baptist but says that he is one-eighth Jewish through his great-grandfather.

Mrs Fray said that her husband's recollection of the meeting, which lasted about an hour, was accurate. Mr McDonald said: "I don't know what provoked it or what. I just remember that one little comment."

Polls consistently show that Mr Lazio, a previously obscure congressman from Long Island, is neck-and-neck with Mrs Clinton.

He entered the race in place of the much better known New York mayor Rudy Giuliani, who withdrew because of health and marital problems.

Mrs Clinton has already had trouble with the Jewish vote and was booed when she took part in an Israel Day parade in Manhattan.

She was criticised last year during a visit to the West Bank for failing to rebuke Yasser Arafat's wife immediately for saying that Israel was poisoning Palestinian children.